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Intersexes seem to crop up now and then in many dipteran families. In the Empididae, they are very common in a few species. On one occasion I netted several thousand in a 'swarm' of Hilara monedula (amongst which were a very few real males or females).
I also recall that there is a Muscid (?) in the Faroe Islands which produces large numbers of intersexes in some years
I think I read somewhere that large scale occurrence of intersexes is a limiting mechanism which decreases reproductive capacity at times when population outstrips resources. Not sure hoow this would work in practice even though it does seem to make good sense for such a mechanism to exist
Hope this helps a bit